In some cases, the PrimaryArtist for a track (or possibly even for a release) is unknown, be it because the sleeve doesn't provide the details, or because no one knows who was the performer.
Such tracks (or releases) should be credited to [unknown].
[unknown] is a SpecialPurposeArtist.
For details about when to use (and not use) [unknown], refer to UnknownArtistStyle.
Please note that [unknown] must not be used as a lazy easy solution: you are expected to at least do some minimal researches to find out the proper performers, and only use [unknown] if the information really can't be found.
There also exists other informally used SpecialPurposeArtist
s, which may be more appropriate than [unknown] depending on the context.